1. Field of the Invention
The present invention relates to an engine fuel control system and more particularly to an air fuel ratio control system for an internal combustion engine. More specifically, the present invention pertains to an engine fuel control system wherein the actual fuel air ratio is detected in terms of the concentration of a composition in the engine exhaust gas and the fuel supply is adjusted in accordance with the thus detected actual air fuel ratio to obtain a desired ratio.
2. Description of the Prior Art
In Japanese patent application No. 56-156940 filed on Oct. 3, 1981 and disclosed for public inspection on Apr. 8, 1983 under the disclosure number 58-59321, there is disclosed an engine fuel control system which has an air fuel ratio detector comprised of an oxygen concentration sensor located in the engine exhaust passage to detect the oxygen concentration in the exhaust gas. A feedback system is provided to transmit the detection signal from the detector to a control circuit wherein the signal is compared with a reference value which is provided for each specific engine operating condition to represent a desired air fuel ratio and fuel supply is adjusted in accordance with the difference between the actual air fuel ratio signal and the reference value. Japanese patent application No. 58-113778 filed on June 24, 1983 and disclosed on Jan. 12, 1985 for public inspection under the disclosure number 60-6036 discloses a similar fuel control system. The U.S. Pat. No. 4,089,313 is also referred to as showing an engine fuel control system having an air fuel ratio detector for detecting the concentration of a composition in the exhaust gas and a controller for producing a correction signal by comparing the signal from the air fuel ratio detector with a reference signal.
In these types of fuel control systems, it is desirable to determine the feedback control factor as small as possible to prevent or suppress huntings. However, where the feedback control factor is too small, the response of the control will become dissatisfactory particularly when the engine operating condition is changed from one state to another. In view of obtaining a satisfactory control response, the conventional fuel control systems have been such that the feedback control factor is of a relatively large value so that a desired air fuel ratio is quickly reached when the engine operating condition is changed.
It should however be noted that recent engine fuel control systems are such that an air fuel mixture leaner than stoichiometric ratio is usually established for normal operations and the mixture is enriched for specific engine operating conditions such as an acceleration and a high power output operating region. In case where the engine operating condition is changed from a high power output region requiring a relatively rich mixture to a normal operating condition wherein a relatively lean mixture is provided, there is a high possibility that the mixture becomes excessively lean possibly causing misfire and/or output torque fluctuations due to possible hunting if the feedback control factor is of a relatively large value. To the contrary, in case where the engine operating condition is changed from a normal operating condition to a high output power region, it is desirable to establish a relatively rich mixture for the high power operation. In the conventional fuel control systems, however, it has been impossible to meet the requirements in both conditions.